with steel taps or hobnails. Oxygen-permeatedclothing will burn vigorouslya most painful wayto die.11. When servicing a liquid oxygen system,ensure that only oxygen conforming to specifica-tion MIL-0-27210D is used. Oxygen procuredunder Federal Specification BB-0-925A is intendedfor technical use and should NOT be used in air-craft oxygen systems.12. After the completion of repairs, alwaysperform an operational check of the system andmake the required tests to ensure that the oxygenis safe for use by the pilot and crew.ONBOARD OXYGENGENERATING SYSTEMLearning Objective: Identify the systemcomponents and operation of the onboardoxygen generating system.The onboard oxygen generating system(OBOGS) is an alternative to liquid oxygen(LOX). When compared to a LOX system, theOBOGS has several advantages. First, itsavailability y may be as high as 99 percent. Thereis no requirement for depot-level maintenance.The OBOGS has no daily service requirements,and scheduled preventive maintenance occurs at2,000 hours. Incorporation of the OBOGSeliminates the need to store and transport LOX.Additionally, it eliminates the need for LOXsupport equipment. The potential for accidentsrelated to LOX and high-pressure gases is greatlyreduced.SYSTEM COMPONENTSThe basic components of the OBOGS are theconcentrator, oxygen monitor, and oxygenbreathing regulator. The concentrator producesan oxygen-rich gas by processing engine bleed airthrough two sieve beds. The oxygen monitorsenses the partial pressure of the gas and, ifnecessary, provides a low-pressure warning to thepilot. The oxygen regulator is a positive pressureregulator.SYSTEM OPERATIONThe OBOGS, shown in figure 4-16, receivesengine bleed air from the outlet of the air-conditioning heat exchanger. The partially cooledair passes through an air temperature sensor toa pressure reducer assembly. The air is then routedFigure 4-16.Onboard Oxygen Generating System(OBOGS) schematic.to the concentrator. The concentrator has a rotaryvalve that alternates the airflow over the molecularsieve beds. The sieve beds absorb the nitrogen andallow the oxygen and argon to pass through. TWOmolecular sieve beds are used in the concentratorso that while one bed is absorbing, the other isdesorbing (releasing) nitrogen. This method allowsa continuous flow of oxygen to the system. Afterthe concentrator, the oxygen flows to a plenumassembly that acts as a surge tank and anaccumulator. The plenum also functions asa heat exchanger to heat or cool the oxygen toapproximately cockpit temperature. Before theoxygen reaches the oxygen regulator, the oxygenperformance monitor senses the partial pressureof the gas and, if necessary, provides a signal tothe pilot whenever the pressure exceeds prescribedlimits. The oxygen then flows through theregulator to the pilots mask.4-25